CDummyCache does not cache anything. It is provided so that one can always configure
a 'cache' application component and he does not need to check if Yii::app()->cache is null or not.
By replacing CDummyCache with some other cache component, one can quickly switch from
non-caching mode to caching mode.

Retrieves multiple values from cache with the specified keys.
Some caches (such as memcache, apc) allow retrieving multiple cached values at one time,
which may improve the performance since it reduces the communication cost.
In case a cache doesn't support this feature natively, it will be simulated by this method.

Stores the value identified by a key into cache.
If the cache already contains such a key, the existing value will be
replaced with the new ones. To add expiration and dependencies, use the set() method.
This method is required by the interface ArrayAccess.